Iridium oxide (IrO x ) is one of the best catalysts for the aqueous oxygen evolution reaction (OER), and its activity is greatly impacted by surface characteristics. By reactively sputtering in a high O 2 flow-rate environment, vertically oriented IrO x nanoplatelets grow several hundred nanometers high exhibiting large surface areas and antireflective optical properties across the visible spectrum. The nanoplatelet IrO x surface is electrochemically compared to other morphologies of IrO x surfaces for OER activity. It was found that the nanoplatelet IrO x surface outperforms all other tested morphologies and planar Ir-metal by exhibiting higher currents and lower overpotentials. Longevity testing of catalytic activity shows that the nanoplatelet surface is more stable in a wide pH range. Characterization with X-ray diffraction and X-ray photoelectron spectroscopy shows that the stoichiometry and oxidation states are similar between the different morphologies of IrO x , but the preferred crystallographic orientation of the rutile IrO x film changes at higher O 2 flow rates. The change from a (110) to a (101) growth direction corresponds with higher OER activity. Nanoplatelet IrO x films are therefore found to expose active sites preferable for the OER, and when combined with their antireflective properties these surfaces are promising for solar water-splitting applications.

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